That really depends. If it was the victim of aggression and you treated it for any injuries, it's technically a hospital tank. If you've done the above, then it would depend on what meds you used, how long they're "prescribed" for, if you completed the treatment, and the fish is back to regular health.
One step further to discuss is if it was the victim, where is the criminal ? If the "criminal" is still in the tank, you may have an issue upon reintroducing the "victim". Therefore, when you put the "victim" back in, the "criminal" may have to come out so things can settle back down. Otherwise history can repeat itself and you're back to the original problem.
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